1. Field of the Invention
The present invention relates to an apparatus for continuously producing a substrate sheet for optical recording mediums used in optically recording and reproducing information using a laser beam or the like. It also relates to a process for continuously producing a substrate sheet for optical recording mediums by the use of the apparatus. The present invention further relates to an apparatus for producing an optical recording medium, and a process for producing an optical recording medium by the use of the apparatus.
2. Related Background Art
In optical recording mediums such as optical disks and optical cards, optically detectable minute pits of several pm, for example, from 1 .mu.m to 10 .mu.m, are formed in a track on a thin recording layer provided on a substrate, and thus information can be recorded in a high density. In such optical recording mediums, a laser beam must be scanned along the track when the information is recorded and reproduced. For this purpose, a substrate with guide grooves for tracking is commonly used.
Methods of preparing such guide grooves or a substrate provided with guide grooves are known to include a compression molding process in which a softened plastic material is pressed using a stamper followed by curing, an injection molding process in which a molten plastic material is injected into a mold provided with e stamper followed by curing, and a 2P (photo-polymerization) process in which guide grooves are transferred from a stamper by the use of a photopolymer. Of these, the 2P process is one of superior processes for preparing substrates in view of the advantages that guide grooves can be readily transferred, substrates have a good solvent resistance, and substrates can be made to have less optical distortion.
More specifically, the superior solvent resistance of a substrate is advantageous when a recording layer is formed by coating, using an organic material as a recording material. For example, when a substrate is prepared by the 2P process, it is possible to use a solvent that can not be used in a substrate comprised of usual plastics such as acrylate and polycarbonate because of its power to dissolve the surface of the substrate. Hence, it becomes possible to use a recording material having a good solubility to the solvent, so that the recording material can be selected from a vast range of materials. The 2P process also has a great advantage that it requires only a very little investment in equipment, compared with other processes.
Compared with other processes, however, the 2P process requires a longer production time per sheet of substrates, and has been involved in the problem that it is not suited for mass-production. Under such circumstances, a proposal, as disclosed in Japanese National Publication (of translated version) No. 62-506504, has been made on a technique by which, using a master on which patterns of pits or grooves have been formed, the patterns are transferred and formed on a film substrate so that substrates for film-like optical recording mediums can be continuously produced.
A conventional method for producing substrates for optical recording mediums will be described below with reference to the accompanying drawing. FIG. 4 illustrates the structure of an apparatus used in forming guide grooves on a substrate according to a conventional method. In FIG. 4, a substrate sheet 1 fed from feed rolls 2 is delivered along the circumference of a roll stamper 3 on which guide grooves or preformat patterns corresponding with information are formed. A nip roll 8 has the function of pressing the substrate sheet 1 against the surface of the roll stamper 3. A resin tank 10 containing a liquid ultraviolet-curable resin 9, curable as a result of exposure to ultraviolet rays, is provided beneath the roll stamper 3. In this resin tank 10, a coating roll 11 is provided which rotates in pressure contact with the roll stamper 3. The liquid ultraviolet-curable resin 9 is coated by this coating roll 11 on the surface of the roll stamper 3. The ultraviolet-curable resin 9 thus coated is held between the substrate sheet 1 and the roll stamper 3 by the action of the nip roll 8. An ultraviolet lamp 13 is provided above the nip roll 8, and the ultraviolet-curable resin 9 held between the substrate sheet 1 and the roll stamper 3 is irradiated with ultraviolet rays so that the ultraviolet-curable resin is cured. A delivery roll 14 has the function of peeling from the roll stamper 3 the substrate sheet 1 and the layer of the ultraviolet-curable resin having been cured in a fixed state to the substrate sheet. Thus, preformat patterns of optical disks are transferred to the substrate sheet.
When, however, the ultraviolet-curable resin and the substrate are laminated according to the above method, the coating surface of the resin and the substrate are in liquid-solid contact, and there has been involved in the problem that bubbles tend to be included when they are brought into contact.
Moreover, guide grooves of the roll stamper or preformat patterns corresponding with information are required to be formed in an accuracy of a submicroscopic order. It, however, has been not so easy to eliminate, in a greater accuracy, rotational irregularities of a DC motor for diving the roll stamper and rotational irregularities caused by gears. This has been one of the causes of the troubles that may bring about eccentricities, defects and recording errors of recording mediums.